1. Yeah, I know what you mean. The only reason that it's very confusing to me is that you can literally pick up about 95% of people's variant stackings just through observing their facial expressions, body language, and overall energy. You can literally pick it up through vibe, but I don't really have the vibe of an sp/sx. They come off as much more dark, moody, intense, and detached. Here's a piece of starters literature on reading people's instincts through vibe:
However, there are some Sp/sx people that sort of have a So variant vibe to them like Hugh Heffner and George Clooney. I think that Fe can be confused for an So variant though.
2. Yes, I agree, I can't be Sx last, but I know that I'm not Sx first either. Sx first people come off as crazily intense and I don't have that fierce intensity to me. Also, I don't really relate to what most Sp/so and so/sp say. Most say that they come off as completely attached and closeness, intensity, and intimacy are quite foreign to them. I mean, don't get me wrong, many so/sp and sp/so people enjoy sex, but it doesn't have the intensity that it has for the people who aren't sx last. Plus, people that are sx last don't tend to get attached to people as easily as those that aren't sx last.